The invention relates to a measuring method for measuring the viscosity of an essentially non-compressible measuring medium with a measuring device comprising a first container, wherein the measuring medium can leak from the first container via a capillary and via an outlet opening of the capillary.
Such a measuring method and such a measuring device are known from document AT 508 276 B1. In the measuring method disclosed in this document, the following process steps are performed for measuring the dynamic viscosity η as a physical quantity of an essentially non-compressible liquid measuring medium:
in a first process step, the measuring medium is introduced into a container filled with a compressible medium, whereupon the measuring medium occupies a partial volume of the total volume of the container and, wherein
in a second process step, an initial pressure of the compressible medium is measured and, wherein
in a third process step, the total volume of the container is changed by a predetermined changing volume ΔV and, wherein
in a fourth process step, a changing internal pressure of the compressible medium caused by the change in volume is measured in the container and, wherein
in a fifth process step, the liquid measuring medium flows through at least one opening in the container through a capillary, wherein the changing internal pressure is measured at at least one measurement time and wherein, in the second process step, the initial pressure of the compressible medium surrounding the liquid measuring medium after the discharge from the capillary is measured, wherein the measurement of the dynamic viscosity η of the liquid measuring medium is carried out by evaluating the measured data by means of the Hagen-Poiseuille law.
Accordingly, in the measuring method according to patent AT 508 276 B1, the time Δt is measured which is required by a certain partial volume ΔVF of the measuring medium for flowing through the capillary in order to determine therefrom the dynamic viscosity η via the equationη=Δt*RK4*π*Δp/(8*LK*ΔVF)obtained by transforming the Hagen-Poiseuille lawΔVF/Δt=+π*RK4*Δp/(8*LK*η).
As per the measuring method according to patent AT 508 276 B1, two measuring operations are necessary for determining the kinematic viscosity ν, wherein, from the combination of the equation(ΔVF/Δt)v,o=+π*DK4*|Δp|/(128*LK*η)+π*DK4*(ρ*g*h)*sin(α)/(128*LK*η)for the measuring medium pressed downward with the equation(ΔVF/Δt)v,o=−π*DK4*|Δp|/(128*LK*η)+π*DK4*(ρ*g*h)*sin(α)/(128*LK*η)for the measuring medium sucked upward, the equation(ΔVF/Δt)v,u−(ΔVF/Δt)v,o=2*π*DK4*Δp/(128*LK*η)for determining the dynamic viscosity and the equation(ΔVF/Δt)v,u+(ΔVF/Δt)v,o=2*π*DK4*(ρ*g*h)sin(α)/(128*LK*η)for determining the density are obtained, and by means of the equationν=η/ρthe kinematic viscosity ν is obtained.
It has turned out to be a disadvantage of the known measuring method and the known measuring device that the measuring apparatus with the capillary arranged horizontally for measuring the dynamic viscosity must be set up horizontally with great precision so as not to obtain any measuring errors. Indeed, the dynamic viscosity is ascertained by adjusting a pressure difference at a capillary lying horizontally, whereas the kinematic viscosity is ascertained by determining the volume flow caused by gravity in a vertically positioned capillary. Hence, if the known measuring apparatus is not set up precisely horizontally, the capillary will also not be located precisely horizontally, for which reason a portion of the kinematic viscosity will be measured along with the determination of the dynamic viscosity, resulting in a measuring error.
In the known measuring apparatus, it has furthermore turned out to be detrimental to the measurement of the kinematic viscosity that                1. two consecutive measurements are necessary, which causes the measurement time to double and therefore also increases the total error of the measurement,        2. Δp must be constant and of exactly the same size in the two process steps “pressing” and “sucking”, but must have opposite signs, which cannot reasonably be accomplished especially in the method with a Δp changing over time,        3. depending on the design of the measuring cell, “pressing” or “sucking” would be more convenient for the measurement, however, in the present method, both process steps must be employed for measuring the kinematic viscosity.        